Remove reader-mode heuristics entirely; explicit per-feed opt-in only

Reader mode was still switching kottke articles even after the v3
opt-out and the "un-hide only if thin" fallback -- both were still
heuristic (content-length-based), and kottke's posts are routinely short
by design, not paywalled, so the heuristics kept firing on ordinary
content. Removed the auto-extract-on-thin-content logic entirely: reader
mode now NEVER turns on by itself, under any condition.

- FeedEntity.readerModeDisabled -> readerModeEnabled (opt-in, default off
  for every feed -- schema v4, full table rebuild since Room's migration
  validation requires an exact column match and a bare ADD COLUMN left
  the old column orphaned; reproduced live as a crash, "Migration didn't
  properly handle: feeds").
- The reader's toggle icon now sets reader mode for the article's WHOLE
  FEED, persists it immediately, and confirms via snackbar ("Reader mode
  enabled/disabled for X") -- both in the reader screen and the Feeds
  list's own toggle. This is the only way reader mode ever turns on.
- ReaderExtractor: replaced the "un-hide everything if thin" paywall
  fallback (which is what pulled kottke's site footer into the result --
  confirmed live, twice) with one that only un-hides elements whose
  class/id actually names them as paywall/premium/subscriber content
  (West Hawaii Today's hidden div is literally class="paywall
  paywall_<id>"). This turned out not to be the kottke bug's actual cause
  either -- see below -- but it's a real, independent correctness
  improvement over the previous blanket version, so it stays.
- Also added: noscript-wrapped lazy-load image fallback (a common
  pattern pairs a placeholder <img> with a real one inside <noscript>,
  which our JS-enabled extraction WebView never parses into DOM per the
  HTML spec) and lazy data-src/srcset resolution generally.

Root cause of the kottke footer bug, found by fetching and inspecting
the actual page structure rather than guessing again: it's not a
CSS-hidden-element issue at all. kottke's real post content lives in a
plain, undecorated <div class="post"> that's a sibling (not ancestor) of
an ads container and a "Socials & More" footer widget, all under one
shared <div class="grid-container">. Because kottke's posts are so
short, Readability's generic paragraph-density scoring doesn't find
enough signal in .post alone and climbs to a broader container that
happens to include the neighboring footer. No un-hide logic is involved
at any point -- this reproduces identically on the plain, unmodified
Readability pass. This is a genuine limit of heuristic content
extraction on very short-form posts, not something to keep patching with
more heuristics. Reset kottke's setting back to off (its correct
default) after testing.

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